Bi-directional DC-DC converter

ABSTRACT

The present invention relates to a bi-directional DC-DC converter comprising: a first terminal, a second terminal, a transformer circuit, a first high voltage side coupled to said first terminal, and a second low voltage side coupled to said second terminal; wherein said first high voltage side and said second low voltage side are coupled to each other by means of said transformer circuit, and said first high voltage side comprises a resonant tank circuit coupled between a first bridge circuit of said first high voltage side and a high voltage side of said transformer circuit. Furthermore, the invention also relates to a system comprising at least two such bi-directional DC-DC converters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP2014/065643, filed on Jul. 21, 2014, which is hereby incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a bi-directional DC-DC converter.Furthermore, the invention also relates to a system comprising at leasttwo such bi-directional DC-DC converters.

BACKGROUND

The developing trends of Isolated Bidirectional Direct Current-DirectCurrent (DC-DC) Converters (IBDC) are Wide Input-Wide Output (WIWO)voltage for very high efficiency, high power density and low cost. Theresonant DC-DC converters are suitable technology to achieve highefficiency due to its intrinsic feature to achieve soft switching (ZeroVoltage Switching, ZVS, and Zero Current Switching, ZCS). Furthermore,it is possible in these circuits to increase the switching frequency inorder to reduce the size of the reactive components.

Common and widely used bidirectional DC-DC converters found in theindustry today are the Dual Active Bridge (DAB) and resonant convertersdue to their availability to achieve high efficiency.

However, there are still remaining drawbacks regarding the conventionalresonant converters at bidirectional operation (i.e. forward- andreverse-mode), e.g. mainly the voltage gain characteristic at reversemode of operation. Furthermore, the high AC-current at the low voltageside of the output filter resulting in high power losses and largevolume of the filter if the current technology is going to be used.

With the described bidirectional topological circuits according toconventional solutions the current stress on the resonant components onthe low voltage side is high and compromises the efficiency of theconverter.

Also, with the described bidirectional topological circuits according toconventional solutions it is not possible to achieve WIWO voltage andhigh efficiency. Moreover, it is very hard to get new topologicalcircuits with reduced number of the active components (controlledsemiconductors) where high reliability and performance in bidirectionalenergy conversion systems are required.

SUMMARY

An objective of the invention is to provide a concept which mitigates orsolves the drawbacks and problems of conventional solutions.

Another objective of the present invention is to provide improvedbi-directional converters for WIWO voltage applications in powersystems.

According to a first aspect of the present invention, the abovementioned and other objective is achieved with a bi-directional DC-DCconverter comprising:

a first terminal circuit,

a second terminal circuit,

a transformer circuit,

a first high voltage side coupled to said first terminal circuit, and

a second low voltage side coupled to said second terminal circuit;wherein

said first high voltage side and said second low voltage side arecoupled to each other by means of said transformer circuit, and

said first high voltage side comprises a resonant tank circuit coupledbetween a first bridge circuit of said first high voltage side and ahigh voltage side of said transformer circuit.

The bridge circuits of the present converter may comprise activeswitches according to an implementation form of the first aspect.

With converters according to embodiments of the present invention veryhigh variation of the input and output voltage, narrow frequencyvariation for voltage regulation, high efficiency, high power densityand low cost can be achieved due the at least the following points. Thepresent converter has simplified and more efficient layout due to theplacement of the resonant tank on the high voltage side. This will alsoreduce the current stress and consequently the losses of the converter.

Furthermore, no energy storage elements in the low voltage side of theconverter are needed in order to get ZVS. Embodiments of the presentinvention can provide ZVS and ZCS in both directions of the converter.

Also, increased reliability is provided due to reduced number of thesynchronous drivers for the low voltage side semiconductors but also dueto the common reference that can be used.

The internal energy consumption needed is also reduced with the presentcircuit layout which will increase the efficiency of the convertersaccording to the present invention compared to conventional converters.

According to a first implementation form of the first aspect as such,said resonant tank circuit comprises: a first branch comprising a firstcapacitor C_(r1) and a first inductor L_(r1) coupled in series with eachother, a second capacitor C_(r2) and a second inductor L_(r2); whereinsaid first branch, said second inductor L_(r2) and said second capacitorare coupled to a common node; wherein said second capacitor C_(r2) iscoupled between said common node and a first terminal of said highvoltage side of said transformer circuit; wherein said second inductorL_(r2) is coupled between said common node (C) and a second terminal ofsaid high voltage side of said transformer circuit.

This can be denoted as a Capacitor-Inductor-Inductor-Capacitor (CLLC)type resonant tank. Therefore, reduced number of the activesemiconductors at high voltage side and low voltage side are needed.

According to a second implementation form of the first implementationform of the first aspect,

a first terminal of the first capacitor C_(r1) forms a first(connection) terminal of said resonant tank circuit,

a second terminal of the first capacitor C_(r1) is connected to a firstterminal of the first inductor L_(r1);

a second terminal of the first inductor L_(r1) is connected to a firstterminal of the second capacitor C_(r2) and to a first terminal of thesecond inductor L_(r2);

a second terminal of the second inductor L_(r2) forms a third(connection) terminal of said resonant tank circuit;

a second terminal of the second capacitor C_(r2) forms a second(connection) terminal of said resonant tank circuit.

According to a third implementation form of the second implementationform of the first aspect,

said first terminal of said resonant tank circuit and said thirdterminal of said resonant tank circuit are connected to said firstbridge circuit; and

said second terminal of said resonant tank circuit and said thirdterminal of said resonant tank circuit are connected to said highvoltage side of said transformer circuit.

According to a fourth implementation form of the first or secondimplementation forms of the first aspect, said first bridge circuit is afull bridge and said second low voltage side comprises a further fullbridge coupled to a low voltage side of said transformer circuit, orsaid first bridge circuit is a half bridge, said second low voltage sidecomprises a push-pull circuit connected to the low voltage side of thetransformer circuit and said transformer circuit comprises on its lowvoltage side a second winding comprising a center tap, or said firstbridge circuit is a half bridge and said second low voltage sidecomprises a push-pull circuit with an autotransformer connected to thelow voltage side of the transformer circuit. Hence, the present resonanttank can be added to any converter topology for different applications.

According to a fifth implementation form of the fourth implementation ofthe first aspect, said resonant tank circuit comprises: a first branchcomprising a first capacitor C_(r1) and a first inductor L_(r1) coupledin series with each other, a second branch comprising a second inductorL_(r2), a second capacitor C_(r2) coupled in series with each other, athird branch comprising a third capacitor C_(r3) and a third inductorL_(r3) coupled in series with each other; wherein said first branch,second branch and third branch are coupled to a common node (C); whereinsaid second branch is coupled between said common node and a firstterminal of said high voltage side of said transformer circuit; andwherein said third branch is coupled between said common node and asecond terminal of said high voltage side of said transformer circuit.This can be denoted as anInductor-Capacitor-Inductor-Capacitor-Inductor-Capacitor or 3LC typeresonant tank. Therefore, reduced number of the active semiconductors athigh voltage side and low voltage side are needed. Further, the voltagegain characteristic is greater than 1, only with passive components andboost and buck mode of operation is possible.

According to a sixth implementation form of the fifth implementationform of the first aspect,

a first terminal of the first capacitor C_(r1) forms a first(connection) terminal of said resonant tank circuit;

a second terminal of the first capacitor C_(r1) is connected to a firstterminal of the first inductor L_(r1);

a second terminal of the first inductor L_(r1) is connected to a firstterminal of the second inductor L_(r2) and to a first terminal of thethird inductor L_(r3);

a second terminal of the second inductor L_(r2) is connected to a firstterminal of the second capacitor C_(r2);

a second terminal of the second capacitor C_(r2) forms a second(connection) terminal of said resonant tank circuit;

a second terminal of the third inductor L_(r3) is connected to a firstterminal of the third capacitor C_(r3);

a second terminal of the third capacitor C_(r3) forms a third(connection) terminal of said resonant tank circuit.

According to a seventh implementation form of the sixth implementationform of the first aspect,

said first (connection) terminal of said resonant tank circuit and saidthird (connection) terminal of said resonant tank circuit are connectedto said first bridge circuit; and

said second (connection) terminal of said resonant tank circuit and saidthird (connection) terminal of said resonant tank circuit are connectedto the high voltage side of said transformer.

According to an eighth implementation form of any of the fifth toseventh implementation forms of the first aspect, said first bridgecircuit is a full bridge and said second low voltage side comprises afurther full bridge coupled to a low voltage side of said transformercircuit, or said first bridge circuit is a half bridge and said secondlow voltage side comprises a full bridge coupled to a low voltage sideof said transformer circuit, or said first bridge circuit is a halfbridge, said second low voltage side comprises a push-pull circuitconnected to the low voltage side of the transformer circuit and saidtransformer circuit comprises on its low voltage side a second windingcomprising a center tap, or said first bridge circuit is a half bridgeand said second low voltage side comprises a push-pull circuit with anautotransformer connected to the low voltage side of the transformercircuit. Hence, the present resonant tank circuit can be added to anyconverter topology for different applications.

According to a ninth implementation form of any of the fifth to eightimplementation forms of the first aspect, at least two of said firstinductor L_(r1), said second inductor L_(r2) and said third inductorL_(r3) are magnetically coupled to each other in one common magneticcore. Thereby, the number of components in the resonant tank circuit canbe reduced.

According to a tenth implementation form of any of the implementationforms of the first aspect or the first aspect as such, a second filteris coupled between a positive and a negative terminal of the secondterminal circuit. Thereby noise can be removed in the low voltage sideof the converter.

According to an eleventh implementation form of any of theimplementation forms of the first aspect or the first aspect as such, afirst filter is coupled in parallel with said first terminal and saidfirst bridge circuit. Thereby noise can be removed in the high voltageside of the converter.

According to a second aspect of the invention, the above mentioned andother objective is achieved with a bi-directional DC-DC converter systemcomprising two or more bi-directional DC-DC converters according to thefirst aspect or any implementation form of the first aspect, whereinsaid two or more bi-directional DC-DC converters are interleaved witheach other, i.e. the bi-directional DC-DC converters are coupled witheach other in different configurations.

Interleaving is to operate two or more DC-DC converters in parallel andto operate the switches of the bridge circuits of each respective DC-DCconverter with phase difference with respect to each other. Thereby, theresultant ripple current in the input and the output of the interleavedsystem can be minimized.

Interleaving two or more of the present converters is preferred for highpower applications. Further, interleaving two or more converters reducesthe number of capacitors needed for the output filter whenphase-shifting control is used. It is also realized that the presentconverters can be interleaved in a variety of different serial andparallel configurations well known in the art.

According to a first implementation form of the second aspect as such,said first high voltage sides of said two or more bi-directional DC-DCconverters are coupled in series with each other.

According to a second implementation form of the first implementationform of the second aspect or the second aspect as such, said first highvoltage sides of said two or more bi-directional DC-DC converters arecoupled in parallel with each other.

According to a third implementation form of the first or secondimplementation forms of the second aspect or the second aspect as such,said second low voltage sides of said two or more bi-directional DC-DCconverters are coupled in series with each other.

According to a fourth implementation form of any of the first to thirdimplementation forms of the second aspect or the second aspect as such,said second low voltage sides of said two or more bi-directional DC-DCconverters are coupled in parallel with each other.

A further aspect of the present invention relates to an electricalcircuit comprising two or more coupling nodes (or terminals) forcoupling to other electrical circuits and two or more inductors, whereinsaid two or more inductors are magnetically coupled to each other in onecommon magnetic core. Thereby, the number of inductive components in theelectrical circuit and also the manufacturing costs are reduced.

It should be noted that further applications and advantages of thepresent converter and system will be apparent from the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to clarify and explain differentembodiments of the present invention, in which:

FIG. 1 shows a bi-directional DC-DC converter according to an embodimentof the present invention;

FIG. 2 shows a CLLC bi-directional DC-DC converter according to anembodiment of the present invention;

FIGS. 3a and 3b show voltage gain characteristics for forward andreverse mode for the CLLC bi-directional DC-DC converter as shown inFIG. 2;

FIGS. 4a-4c show other further CLLC bi-directional DC-DC convertersaccording to embodiments of the present invention;

FIG. 5 shows a 3LC bi-directional DC-DC converter according to anembodiment of the present invention;

FIGS. 6a and 6b show voltage gain characteristics for forward andreverse mode for the 3LC bi-directional DC-DC converter as shown in FIG.5;

FIGS. 7a-7c show other further 3LC bi-directional DC-DC converteraccording to embodiments of the present invention;

FIGS. 8a and 8b show two different 3LC resonant tanks as they can beused with the embodiments shown in FIGS. 5 and 7 a-7 c;

FIG. 9 shows a bi-directional DC-DC converter system according to anembodiment of the present invention comprising CLLC bi-directional DC-DCconverters; and

FIG. 10 shows a bi-directional DC-DC converter system according to anembodiment of the present invention comprising 3LC bi-directional DC-DCconverters.

DETAILED DESCRIPTION

FIG. 1 shows a simplified block diagram of a bi-directional DC-DCconverter 100 according to an embodiment of the present invention. Withreference to FIG. 1 the bi-directional DC-DC converter 100 comprises a(first) High Voltage (HV) (e.g. connection) terminal circuit 101 of anHV side 107, a (second) Low Voltage (LV) (e.g. connection) terminalcircuit 103 of an LV side 109 and a transformer circuit 105. The HV side107 is coupled to the first terminal circuit 101 of the DC-DC converter100, and the LV side 109 is coupled to the second terminal circuit 103of the DC-DC converter 100.

Further, the HV side 107 and the LV side 109 are coupled to each otherby means of the mentioned transformer circuit 105. Moreover, the HV side107 comprises a resonant tank circuit 111 coupled between a first bridgecircuit 113 of the HV side 107 and a HV side of the transformer(circuit) 105. The terminal circuits 101 and 103 of the converter 100and the different implementation form of this converter 100 described inthe following typically comprise a positive terminal (for applying orproviding a positive potential) and a negative terminal (e.g. forapplying or providing a negative or GND potential). These positive andnegative terminals are typically connection terminals adapted forconnecting to one or more other devices. In the forward direction (Highvoltage in-Low voltage out) of the converter 100, the first terminalcircuit 101 forms in input of the converter 100 and the second terminalcircuit 103 forms an output of the converter 100. In the reversedirection (Low voltage in-High voltage out) of the converter 100, thesecond terminal circuit 103 forms in input of the converter 100 and thefirst terminal circuit 101 forms an output of the converter 100.

HV side and LV side mean that at the HV side typically the comparativelyhigher voltages are applied/are provided when compared to the LV side.

According to an embodiment of the present invention, the resonant tankcircuit 111 is of Capacitor-Inductor-Inductor-Capacitor (CLLC) type.FIG. 2 shows a bi-directional DC-DC converter 200 according to anembodiment of the present invention with a CLLC resonant tank 111. Thebi-directional DC-DC converter 200 forms a possible implementation formof the bi-directional DC-DC converter 100 as shown in FIG. 1.

In the CLLC bi-directional DC-DC converter 200 an example of a CLLCresonant tank 111 implemented in the HV side 107 of the bi-directionalDC-DC converter is shown in FIG. 2. Among the characteristics of thisresonant tank circuit are the possibility to achieve WIWO voltage rangein forward mode and acceptable voltage gain in reverse mode, but alsohigh efficiency and high power density.

With reference to FIG. 2 the resonant tank circuit 111 according to theCLLC embodiment comprises a first capacitor C_(r1), a first inductorL_(r1), a second capacitor C_(r2), and a second inductor L_(r2).

A first terminal of the first capacitor C_(r1) forms a first(connection) terminal T1 of the CLLC resonant tank circuit 111. A secondterminal of the first capacitor C_(r1) is connected to a first terminalof the first inductor L_(r1). A second terminal of the first inductorL_(r1) is connected to a first terminal of the second capacitor C_(r2)and to a first terminal of the second inductor L_(r2). Further, a secondterminal of the second inductor L_(r2) forms a third (connection)terminal T3 of the resonant tank circuit 111. A second terminal of thesecond capacitor C_(r2) forms a (second) connection terminal T2 of theCLLC resonant tank circuit 111.

Furthermore, the HV side 107 comprises a first full bridge circuit 113coupled between the first HV terminal circuit 101 and the resonant tankcircuit 111.

The first connection terminal T1 of the resonant tank circuit 111 isconnected between third S3 and fourth S4 switches of the first bridgecircuit 113. The third connection terminal T3 of the resonant tankcircuit 111 is connected between first S1 and second S2 switches of thefirst bridge circuit 113. The second connection terminal T2 of theresonant tank circuit 111 is connected to a first terminal of the HVside (e.g. a first ending of a first winding) of the transformer circuit105, and the third connection terminal T3 of the resonant tank circuit111 is connected to a second terminal of the HV side (of a second endingof the first winding) of the transformer circuit 105.

In other words the CLLC resonant circuit 111 according to thisembodiment comprises a first branch comprising a first capacitor C_(r1)and a first inductor L_(r1) coupled in series with each other, a secondcapacitor C_(r2) and a second inductor L_(r2). The first branch, saidsecond inductor L_(r2) and said second capacitor are coupled to a commonnode C. Said second capacitor C_(r2) is coupled between said common nodeC and the first terminal of said high voltage side of said transformercircuit 105. Said second inductor L_(r2) is coupled between said commonnode C and the second terminal of said high voltage side of saidtransformer circuit 105.

The values for the different capacitors and inductors of the presentresonant tank 111 are dependent on the particular application.

The HV side 107 includes the first terminal circuit 101 which isconnected to first and second terminals of a first filter 117implemented as a capacitor C_(HV) in this particular example. In detail,the first filter 117 is connected between the positive terminal and thenegative terminal of the first terminal circuit 101.

The first and second terminals of the first filter 117 are in turnconnected to a positive terminal and a negative terminal of the fullbridge circuit 113, respectively. The full bridge circuit 113 comprisesswitches S1, S2, S3 and S4 implemented as N-Channel Mosfet transistorsin this example. However, other implementations for the switches arepossible too (such as Insulated Gate Bipolar Transistor, IGBT; MetalOxide Silicon Field Effect Transistor, MOSFET; Junction GateField-Effect Transistor, JFET; Gate Turn-off Thyristor, GTO).

Mentioned switches S1, S2, S3 and S4 of the full bridge circuit 113 ofthe HV side are followed by the above described CLLC resonant tankcircuit 111 which in turn is connected to the HV side of the transformercircuit 105. The transformer circuit 105 magnetically couples the HVside 107 and the LV side 109 of the converter device 200.

Further, first and second terminals of the LV side (e.g. endings of asecond winding) of the transformer (circuit) 105 are connected to asecond full bridge circuit 115 of the LV side 109. The second fullbridge circuit 115 includes first Sr₁, second Sr₂, third Sr₃ and fourthSr₄ switches. A positive and a negative connection terminals of thesecond full bridge circuit 115 are connected to first and secondterminals of a second filter 119 of the LV side 109 which in thisexample is implemented as a capacitor C_(LV). Finally, the first andsecond terminals of the second filter 119 are connected to the secondterminal circuit 103 of the present DC-DC converter 200. In detail, thefirst filter 117 is connected between the positive terminal and thenegative terminal of the second terminal circuit 103.

The voltage gain characteristics for both forward (shown in FIG. 3a )and reverse mode (shown in FIG. 3b ) of this particular embodiment inFIG. 2 are shown in FIGS. 3a and 3 b.

The y-axis represents the voltage and the x-axis represents thefrequency. As it can be seen in the graphs of FIGS. 3a and 3b , thenatural resonance frequency of the resonant tank is equal in bothdirections. In the reverse mode, the voltage gain characteristic islimited and highly dependent of the quality factor Q which is dependenton the values of the components in the resonant tank circuit 111 (in theFIGS. 3a and 3b Q=10, 2 and 0.1 are shown). However, this is a designrelated issue and will depend of the choice of the values for thecomponents (parameters) of the resonant tank and the application.

Based on the configuration of the LV side 107 of the above describedconverter, different topological implementation forms of thebi-directional DC-DC converter are possible which are illustrated inFIG. 4a-4c . These implementation forms relate to the configurations ofthe first bridge circuit 113 and the second bridge circuit 105,respectively.

In the converter 210 shown in FIG. 4a , the first bridge circuit 113 inthe HV-side 107 is implemented as a Half Bridge (HB) circuit and thesecond bridge circuit 115 in the LV side 109 is implemented as a FullBridge (FB) circuit.

The HB circuit 113 in the HV side 107 in FIG. 4a comprises first S1 andsecond S2 switches; first CB1 and second CB2 capacitors (CB1 and CB2 maybe part of the resonant tank circuit in certain applications when thefirst bridge circuit has this HB configuration); and first Dc1 andsecond Dc2 clamping diodes. The first connection terminal T1 of theresonant tank circuit 111 is connected between a series connection ofthe first capacitor CB1 in parallel with the first clamping diode Dc1and the second capacitor CB2 in parallel with the second clamping diodeDc2. The third connection terminal T3 of the resonant tank circuit 111is connected between the series connection of the first switch S1 andthe second switch S2.

The FB circuit in the LV side 109 in FIG. 4a is configured in the sameway as the FB circuit in FIG. 2 as described above.

In the converter 220 as shown in FIG. 4b , the first bridge circuit 113in the HV-side 107 is implemented as a Half Bridge circuit (as the onein FIG. 4a ). Furthermore, the LV side 107 of the convert 220 comprisesinstead of a bridge circuit a push pull-autotransformer circuit 116.

The PP-autotransformer circuit 116 in the LV side in FIG. 4b comprisesswitches Sr1, Sr2 and an autotransformer 123. The autotransformer 123has two windings (first and second windings) in one common core. Thefirst winding is connected the first switch Sr1, and the second windingis connected to the second switch Sr2. The midpoint of theautotransformer 123 is connected to a positive terminal of the LV side.The common point of Sr1 and Sr2 are connected to a negative terminal ofthe LV side. Further, the first terminal of the LV side (e.g. a firstending of a second winding) of the transformer 105 is connected betweenthe first winding of the autotransformer 123 and the first switch Sr1.The second terminal of the LV side (e.g. a second ending of the secondwinding) of the transformer 105 is connected between the second windingof the autotransformer 123 and the second switch Sr2.

In the converter 230 shown in FIG. 4c , the first bridge circuit 113 isimplemented as HB circuit. Furthermore, the converter 230 comprises inits LV side instead of a bridge circuit a Push Pull (PP) Circuit 118.The PP circuit 118 comprises a first switch Sr1 and a second switch Sr2.In the embodiment shown in FIG. 4c , the first switch Sr1 and the secondswitch Sr2 are implemented exemplarily as N-channel Mosfets. Thetransformer circuit 105 is implemented as a transformer comprising onits HV side a first winding connected with its first end to the secondterminal T2 and its second end to the third terminal T3 of the resonanttank circuit 111. Furthermore, on LV side the transformer comprises asecond winding having a first end, a center tap and a second end. Afirst terminal of the first switch Sr1 (e.g. a drain terminal) isconnected to the second end of the second winding. A first terminal ofthe second switch Sr2 (e.g. a drain terminal) is connected to the firstend of the second winding. Second terminals (e.g. source terminals) ofthe first switch Sr1 and second switch Sr2 are connected together to anegative terminal of the second terminal circuit 103. Furthermore, thecenter tap of the transformer is connected to the positive terminal ofthe second terminal circuit 103. A second filter C_(LV) is connectedbetween the negative and positive terminal of the second terminalcircuit 103.

According to an embodiment of the present invention, the tank circuit111 is of three Inductor-Capacitor type, i.e.Inductor-Capacitor-Inductor-Capacitor-Inductor-Capacitor, denoted 3LC inthis disclosure. FIG. 5 shows such a 3LC resonant tank 111 implementedin the HV side 107 of a bi-directional DC-DC converter 300 according toan embodiment of the to the present invention, which forms a possibleimplementation form of the converter 100. Among the characteristics ofthis resonant tank circuit 111 are the possibility to achieve suitablevoltage gain in reverse mode, but also high efficiency and high powerdensity. Another important effect is that the WIWO voltage range can beachieved with a very narrow frequency variation.

From left in FIG. 5 the HV side 107 includes a first terminal circuit101 of the HV side which is connected to first and second terminals of afirst filter 117 implemented as a capacitor in this example, C_(HV). Thefirst and second terminals of first filter 117 are in turn connected toa positive terminal and a negative terminal of the FB circuit 113 asdescribed above. Mentioned switches S1, S2, S3 and S4 of the FB circuit113 of the HV side are followed by the 3LC resonant tank circuit 111.The FB circuit is coupled to connection terminals T1 and T3 of theresonant tank circuit 111. Further, the resonant tank circuit 111 isconnected to the LV side of the transformer circuit 105 via connectionterminals T2 and T3. The transformer circuit 105 magnetically couplesthe HV side 107 and the LV side 109 of the present converter 300.

In the example shown in FIG. 5, the LV side 109 of the converter 300 isimplemented as a push pull circuit 118 (comprising switches Sr₁, Sr₂ incombination with the transformer circuit 105 having the second windingwith center tap (as shown in FIG. 4c ). Hence, the converter 300 differsfrom the converter 230 in that the resonant tank circuit 111 in theconverter 300 is implemented as 3LC resonant tank.

Alternatively also an implementation with a push pull-autotransformercircuit 116 on the LV side 109 would be possible (as shown in FIG. 4b ).

FIG. 8a shows one proposed 3LC resonant tank 111 configuration for usein embodiments of the present invention which comprises a firstcapacitor C_(r1), a first inductor L_(r1), a second inductor L_(r2), asecond capacitor C_(r2), a third inductor L_(r3) and a third capacitorC_(r3). A first terminal of the first capacitor C_(r1) forms a firstconnection terminal T1 of the 3LC resonant tank circuit 111. A secondterminal of the first capacitor C_(r1) is connected to a first terminalof the first inductor L_(r1). A second terminal of the first inductorL_(r1) is connected to a first terminal of the second inductor L_(r2)and to a first terminal of the third inductor L_(r3). A second terminalof the second inductor L_(r2) is connected to a first terminal of thesecond capacitor C_(r2). A second terminal of the second capacitorC_(r2) forms a second connection terminal T2 of the resonant tankcircuit 111. A second terminal of the third inductor L_(r3) is connectedto a first terminal of the third capacitor C_(r3). A second terminal ofthe third capacitor C_(r3) forms a third connection terminal T3 of theresonant tank circuit 111. As can be seen from FIG. 8a , the threeinductors L_(r1), L_(r2), L_(r3) are all connected to a common node C.

In other words the proposed 3LC resonant tank configuration shown inFIG. 8a comprises a first branch comprising a first capacitor C_(r1) anda first inductor L_(r1) coupled in series with each other, a secondbranch comprising a second inductor L_(r2), a second capacitor C_(r2), athird branch comprising a third capacitor C_(r3) and a third inductorL_(r3) coupled in series with each other. The first branch is coupled inseries with the second branch, and the third branch is coupled between acommon node C of the first branch and the second branch and the thirdterminal T3 of the resonant tank circuit 111. The second T2 and thethird terminals T3 of the resonant tank circuit 111 are to be coupled tothe high voltage side of the transformer circuit 105. The values for thedifferent capacitors and inductors of the present resonant tank 111 aredependent on the particular application.

The features of this resonant tank 111 are unique as it increases thevoltage gain for both directions to be greater than 1 which is the gainobtained at the resonant frequency. This feature makes it possible toachieve WIWO voltage variation.

The voltage gain characteristics for both forward and reverse mode areshown in FIGS. 6a (forward mode) and 6 b (reverse mode) for the 3LCbi-directional DC-DC converter 300 as shown in FIG. 5. The y-axisrepresents the voltage and the x-axis represents the frequency. As itcan be seen in the graphs in FIGS. 6a and 6b , the natural resonancefrequency of the tank is equal in both directions. The effect introducedby the parallel LC-network (L_(r3) and C_(r3)) makes the gain to changefrom 0 to infinity in a very sharp way. The final value is depending ofthe quality factor Q (Q=10, 2, 0.1 are shown in FIGS. 6a and 6b ). Thisresult in high gain, but it is also important to mention that thecharacteristics of this converter are the same as the standard LLCresonant tank for both directions. This guarantees that the converterwill always work in the most optimum power transferring point in bothforward and reverse modes.

In FIG. 8b a magnetic integration of the first L_(r1) and the secondL_(r2) inductors of the resonant tank circuit 111 using the same magnetcore is illustrated. This simplifies the building of the 3LC resonanttank circuit 111 of the present converter and also reduces the number ofcomponents. However, all the inductors of the resonant tank circuit 111could be integrated in a single magnetic component. Therefore,embodiments of the present invention also relate to an electricalcircuit 111 comprising two or more coupling nodes for coupling to otherelectrical circuits and two or more inductors, wherein the two or moreinductors are magnetically coupled to each other in one common magneticcore. The present electrical circuit 111 can also be used in otherapplications in which two or more inductors are used.

Based on the 3LC resonant tank 111, we have different convertertopological circuits that are illustrated (additionally to the one shownin FIG. 5) in FIG. 7a-7c according to further embodiments of the presentinvention.

The FB, HB and PP circuits in FIG. 7a-7c are configured in the same wayas the FB, HB and PP circuits in the embodiments shown in FIG. 2, 4 a, 4b. FIG. 7a shows a Full Bridge-Full Bridge implementation for the bridgecircuits 113, 115 as the one in FIG. 2. FIG. 7b shows a Half Bridge-FullBridge implementation for the bridge circuits 113, 115 as the one shownin FIG. 4a . FIG. 7c shows a Half Bridge-Push Pull circuit withAutotransformer implementation for the bridge circuit 113 and LV side109 as the one shown in FIG. 4 b.

For high power applications, interleaving two or more DC-DC convertersof embodiments of the present invention is preferred so as to obtain abi-directional DC-DC converter system.

For instance, one possible configuration is to have series connection ofthe HV sides 107 a, 107 b, . . . , 107 n of the DC-DC converters andparallel connection of the LV sides 109 a, 109 b, 109 n of the DC-DCconverters. This configuration setup is illustrated in the systems inFIGS. 9 and 10, respectively. The single DC-DC converter can be of anytype according to embodiments of the present invention.

Other configurations for the connection of the individual converters thesystem 1000 are: in parallel in the HV side 107 and in series in the LVside 109, in series in the HV side 107 and in series in the LV side 109,and in parallel in the HV side 107 and in parallel in the LV side 109.

FIG. 9 shows a DC-DC converter system 900 according to an embodiment ofthe present invention in which the resonant tank circuit 111 of eachDC-DC converter in the system 900 is of the CLLC type as explainedabove. In detail does FIG. 9 show an interleaving of a plurality ofconverters 200 as shown in FIG. 2. In this system 900, the HV sides 107a . . . 107 n of the converters are connected in series between apositive HV terminal and a negative HV terminal of the system 900. TheLV sides 109 a . . . 109 n of the converters are connected in parallelto a positive LV terminal and a negative LV terminal of the system 900.

FIG. 10 shows a DC-DC converter system 1000 according to an embodimentof the present invention in which the resonant tank circuit 111 of eachconverter in the system 1000 is of the 3LC type as explained above. Indetails does FIG. 10 show an interleaving of a plurality of converters300 as shown in FIG. 5. In this system 1000, the HV sides 107 a . . .107 n of the converters are connected in series between a positive HVterminal and a negative HV terminal of the system 1000. The LV sides 109a . . . 109 n of the converters are connected in parallel to a positiveLV terminal and a negative LV terminal of the system 1000.

Although the examples in FIGS. 9 and 10 showed an interleaving of theDC-DC converters 200 and 300, further embodiments also include aninterleaving the other DC-DC converters introduced in this documents(also in the general form of the DC-DC converter 100).

Finally, it should be understood that the present invention is notlimited to the embodiments described above, but also relates to andincorporates all embodiments within the scope of the appendedindependent claims.

What is claimed is:
 1. A bi-directional DC-DC converter, comprising: afirst terminal circuit; a second terminal circuit; a transformercircuit; a first high voltage side coupled to the first terminalcircuit; and a second low voltage side coupled to the second terminalcircuit; wherein the first high voltage side and the second low voltageside are coupled to each other via the transformer circuit; wherein thefirst high voltage side comprises a resonant tank circuit coupledbetween a first bridge circuit of the first high voltage side and a highvoltage side of the transformer circuit; and wherein the resonant tankcircuit comprises: a first branch comprising a first capacitor and afirst inductor coupled in series; a second branch comprising a secondinductor and a second capacitor coupled in series; and a third branchcomprising a third capacitor and a third inductor coupled in series;wherein the first branch, the second branch and the third branch arecoupled to a common node; wherein the second branch is coupled betweenthe common node and a first terminal of the high voltage side of thetransformer circuit; and wherein the third branch is coupled between thecommon node and a second terminal of the high voltage side of thetransformer circuit.
 2. The bi-directional DC-DC converter according toclaim 1, wherein: a first terminal of the first capacitor forms a firstterminal of the resonant tank circuit; a second terminal of the firstcapacitor is connected to a first terminal of the first inductor L_(r1);a second terminal of the first inductor is connected to a first terminalof the second inductor and to a first terminal of the third inductor ; asecond terminal of the second inductor is connected to a first terminalof the second capacitor; a second terminal of the second capacitor formsa second terminal of the resonant tank circuit; a second terminal of thethird inductor is connected to a first terminal of the third capacitor;and a second terminal of the third capacitor forms a third terminal ofthe resonant tank circuit.
 3. The bi-directional DC-DC converteraccording to claim 2, wherein: the first terminal of the resonant tankcircuit and the third terminal of the resonant tank circuit areconnected to the first bridge circuit; and the second terminal of theresonant tank circuit and the third terminal of the resonant tankcircuit are connected to the high voltage side of the transformercircuit.
 4. The bi-directional DC-DC converter according to claim 1,wherein the first bridge circuit is a full bridge and the second lowvoltage side comprises a further full bridge coupled to a low voltageside of the transformer circuit, or the first bridge circuit is a halfbridge and the second low voltage side comprises a full bridge coupledto a low voltage side of the transformer circuit, or the first bridgecircuit is a half bridge, the second low voltage side comprises apush-pull circuit connected to the low voltage side of the transformercircuit and the transformer circuit comprises on its low voltage side asecond winding comprising a center tap, or the first bridge circuit is ahalf bridge and the second low voltage side comprises a push-pullcircuit with an autotransformer connected to the low voltage side of thetransformer circuit.
 5. The bi-directional DC-DC converter accordingclaim 1, wherein at least two of the first inductor, the second inductorand the third inductor are magnetically coupled to each other in onecommon magnetic core.
 6. The bi-directional DC-DC converter according toclaim 1, further comprising: a second filter coupled between a positiveterminal and a negative terminal of the second terminal circuit.
 7. Thebi-directional DC-DC converter according to claim 1, further comprising:a first filter coupled in parallel with the first bridge circuit.
 8. Abi-directional DC-DC converter system, comprising two or morebi-directional DC-DC converters interleaved with each other, whereineach bi-directional DC-DC converter comprises: a first terminal circuit;a second terminal circuit; a transformer circuit; a first high voltageside coupled to the first terminal circuit; and a second low voltageside coupled to the second terminal circuit; wherein the first highvoltage side and the second low voltage side are coupled to each othervia the transformer circuit; wherein the first high voltage sidecomprises a resonant tank circuit coupled between a first bridge circuitof the first high voltage side and a high voltage side of thetransformer circuit; and wherein the resonant tank circuit comprises: afirst branch comprising a first capacitor and a first inductor coupledin series; a second branch comprising a second inductor and a secondcapacitor coupled in series; and a third branch comprising a thirdcapacitor and a third inductor coupled in series; wherein the firstbranch, the second branch and the third branch are coupled to a commonnode; wherein the second branch is coupled between the common node and afirst terminal of the high voltage side of the transformer circuit; andwherein the third branch is coupled between the common node and a secondterminal of the high voltage side of the transformer circuit.
 9. Thebi-directional DC-DC converter system according to claim 8, wherein thefirst high voltage sides of the two or more bi-directional DC-DCconverters are coupled in series or coupled in parallel.
 10. Thebi-directional DC-DC converter system according to claim 8, wherein thesecond low voltage sides of the two or more bi-directional DC-DCconverters are coupled in series or coupled in parallel.
 11. A resonanttank circuit, comprising: a first branch comprising a first capacitorand a first inductor coupled in series; a second branch comprising asecond inductor and a second capacitor coupled in series; a third branchcomprising a third capacitor and a third inductor coupled in series; andtwo or more coupling nodes for coupling to other electrical circuits;wherein the first branch, the second branch and the third branch arecoupled to a common node; wherein the second branch is coupled betweenthe common node and a first terminal of a high voltage side of atransformer circuit; and wherein the third branch is coupled between thecommon node and a second terminal of the high voltage side of thetransformer circuit.